1. Field of the Invention
The present invention relates to a sheet processing apparatus capable of performing a punch process to punch holes in a sheet and an image forming system having the sheet processing apparatus.
2. Description of the Related Art
In the related art, a sheet processing apparatus capable of performing a punch process to punch holes in a sheet has been combined with an image forming apparatus for improving efficiency of operation to keep or use image-formed sheets by binding with a file or a ring.
With such a sheet processing apparatus, a number of sheets which are punched for binding are stacked on a stack tray. However, since burrs may be generated due to the punch process in sheets, there may be a risk that stack error occurs caused by the burrs of holes.
Accordingly, in the related art, there has been proposed a configuration to prevent the stack error caused by the burrs of holes punched in sheets. For example, a configuration to prevent the stack error caused by hole burrs by switching stack limit number of sheets on the stack tray depending on presence of punch process performing is disclosed in Japanese Patent Application Laid-open No. 11-079536. Specifically, the first stack limit number is selected in the case without the punch process performing and the second stack number which is smaller than the first stack limit number is selected in the case with the punch process performing.
Recently, a sheet processing apparatus capable of punching holes of different number, shape and size with the single sheet processing apparatus by replacing a punch for punching has been proposed in order to be ready for a variety of files and rings. When the number, shape and size of the holes punched in the sheets are different, the shape and size of the burrs becomes different even in a case that the punch process is performed in the same manner. Accordingly, stacking ease of the sheets onto the stack tray remarkably varies.
The influence of hole types (i.e., the number, shape and size) to the stacking ease becomes apparent in a case that a large capacity stacker capable of stacking sheets vertically in the order of five thousands on a single horizontal stack tray is combined with the abovementioned sheet processing apparatus.
Accordingly, in the case that there are two stack limit numbers depending on the presence of the punch process performing as described above, the stack limit number must be set within a range to ensure the stacking ease of the hole type of the worst conditions. For example, it is assumed that sheets with two holes can be stacked in a well-aligned manner up to four thousands and the upper limit number of well-aligned stacking of sheets with thirty holes is one thousand. In this case, the stack limit number has to be set to one thousand even for the sheets with two holes. Accordingly, the performance of the large capacity stacker cannot be exploited, so that the stack tray becomes full frequently. Consequently, downtime is increased and usability is decreased. On the contrary, when the stack limit number of sheets with the punch process performing is set to be four thousands which is the upper limit for the sheets with two holes, interference between the burrs and interference between sheet end portions and the burrs occur at the time of stacking the sheets with thirty holes. In addition, the height difference at the upper surface of the sheets occurs due to overlapping of the burrs. Accordingly, the sheet alignment is not maintained and stacking error occurs. In a worse case, there is a risk to cause paper jamming, stack slipping and the like.